Cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography for the analysis of methadone and its metabolites in serum of heroin addicts

Methadone (MET) metabolism has been largely demonstrated with a high inter-individual variability and, therefore, quantification of MET is very important for therapeutic drug monitoring. A cation-selective exhaustive injection and sweeping MEKC (CSEI-Sweeping) was first developed to analyze MET and its two metabolites, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP), in human serum. After pretreatment, the samples were electrokinetically injected into capillary (10 kV, 500 s) and swept by the separation phosphate buffer (100 mM, pH 4.0) containing 20% tetrahydrofuran and 100 mM SDS at −15 kV. The LODs were 200 pg/mL for MET and EMDP, and 400 pg/mL for EDDP. Ten volunteers were administered MET (5.0–120.0 mg/day) orally for 84 days and serum samples were taken after the daily dose of MET (days 1, 2, 7, 14, 28, 56 and 84) individually. This method was used for monitoring MET and its metabolites in heroin addicts and for pharmacokinetic investigations.